Mucus and Mucins

Mucus is a "slimy" material that coats many epithelial surfaces and is secreted into fluids such as saliva. It is composed chiefly of mucins and inorganic salts suspended in water.

Mucus adheres to many epithelial surfaces, where it serves as a diffusion barrier against contact with noxious substances (e.g. gastric acid, smoke) and as a lubricant to minimize shear stresses; such mucus coatings are particularly prominent on the epithelia of the respiratory, gastrointestinal and genital tracts. Mucus is also an abundant and important component of saliva, giving it virtually unparalleled lubricating properties (try sticking a piece of apple skin between your molars without saliva).

Mucus-secreting cells are widely distributed through the body. Goblet cells are abundant in the epithelium of the gastrointestinal and respiratory tracts, mucous glands in these same organs deliver their products through ducts into the intestine and respiratory tree, and many of the acinar epithelial cells in salivary glands secrete mucus.

Mucins are a family of large, heavily glycosylated proteins. Although some mucins are membrane bound due to the presence of a hydrophobic membrane-spanning domain that favors retention in the plasma membrane, the concentration here is on those mucins that are secreted on mucosal surfaces and saliva.

Mucin genes encode mucin monomers that are synthesized as rod-shaped apomucin cores that are post-translationally modified by exceptionally abundant glycosylation. Two distinctly different regions are found in mature mucins:

The amino- and carboxy-terminal regions are very lightly glycosylated, but rich in cysteines, which are likely involved in establishing disulfide linkages within and among mucin monomers.

A large central region formed of multiple tandem repeats of 10 to 80 residue sequences in which up to half of the amino acids are serine or threonine. This area becomes saturated with hundreds of O-linked oligosaccharides. N-linked oligosaccharides are also found on mucins, but much less abundantly.

The dense "sugar coating" of mucins gives them considerable water-holding capacity and also makes them resistant to proteolysis, which may be important in maintaining mucosal barriers.

Mucins are secreted as massive aggregates with molecular masses of roughly 1 to 10 million Da. Within these aggregates, monomers are linked to one another mostly by non-covalent interactions, although intermolecular disulfide bonds may also play a role in this process.